The invention relates to a portable system for the real time detection and measurement of one or several gases in a gas mix, and particularly atmospheric gases, by absorption spectrometry.
The invention is used in many industrial fields and particularly for industries generating gaseous waste released into the atmosphere such as refineries and thermal power stations, to determine the concentration of one or more toxic or dangerous gases, and in raw materials transformation industries such as the chemical and pharmaceutical industries, to check the atmosphere in which their employees are working. It is also used in atmospheric monitoring applications to determine the content of polluting gases in the atmosphere.
The principle of measuring gases in the atmosphere using the absorption spectrometry technique is well known to those skilled in the art. This technique consists of determining the absorption of an area of the atmosphere. This is done by illuminating the area of the atmosphere to be analyzed by a light source; therefore light passes through this area of the atmosphere. The light signal obtained after passing through this area of the atmosphere is retrieved and quantified on a detector in order to obtain information about the quantity of light transmitted, and consequently the quantity of light absorbed by the area of the atmosphere. The absorption spectrum of the measured gas is then determined. The gas to be measured is usually a known gas, and known gases are characterized spectroscopically by their absorption spectrum. Thus, gases present in the analyzed area of the atmosphere are recognized by comparing their absorption spectrum with the absorption spectrum determined by measurement with known spectra.
Many documents describe processes and devices for detecting gases in the atmosphere. Most of these devices use a laser source emitting a precise wave length. These documents include French patent applications FR-A-2 389 888, FR-A-2 388 261, FR-A-2171 519 and FR-A-2 256 407, and also French patent application FR-A-2 747 192. However, these devices have the disadvantage that they can only be used to measure a single gas selected in advance, since the choice of the laser source and the wave length that will be emitted by this laser source are directly related to the gas to be detected.
Furthermore, there is a device capable of emitting wave lengths that cannot be emitted by conventional laser sources. This device operates on the principle of frequency difference and is described in the following publications: xe2x80x9cMidinfrared CW difference-frequency generation using a synchronous scanning technique for continuous tuning of the full spectral region from 4.7 to 6.5 xcexcmxe2x80x9d, by W. CHEN, J. BURIE and D. BOUCHER, Rev. Sci. Instrum. Vol 67, No. 10, October 1996; xe2x80x9cA novel CW optical Laser-based Difference-frequency infrared Spectrometerxe2x80x9d, by D. BOUCHER, W. CHEN, J. BURIE and P. PEZE, 5th conference on lasers and quantitative optics, Sep. 8-10, 1997, Strasbourg. This device comprises two titanium-sapphire (TiSa) lasers that generate infrared light radiation obtained by the difference between two light radiation beams initially emitted by the two lasers; the difference between the two initial radiation beams is obtained using a double refraction crystal. Different wavelengths can be obtained by varying this difference, so that several gases can be detected. However, this device requires the use of two lasers; therefore it is large and occupies an area of several square meters, and its weight exceeds one tonne. Therefore this device is difficult to move and cannot be used locally in the ambient atmosphere to be analyzed.
The purpose of the invention is to overcome these disadvantages. Consequently, it proposes an optical detection and quantitative measurement system for one or several gases in a gas mix, such as gases contained in the atmosphere. This system is compact and may be moved easily so that local gas analyses can be carried out in confined or semi-open spaces, even within the gas mix to be analyzed.
More precisely, the invention relates to a system for detection and measurement of a gas or several gases making up a gas mix, comprising:
optical means for determining the absorption spectrum of the gas to be measured;
means of storing data for different gases and for processing information output from optical means; and
electrical power supply means for the optical means,
characterized in that the optical means are placed within the gaseous medium to be analyzed and that they comprise:
at least one first and one second laser diode with matchable cavities, emitting light beams with a first and a second wavelength, respectively;
a light beam mixer prism;
a double refraction crystal mounted on a rotating table forming an infrared light beam with a wavelength equal to the resultant of the difference between the first and second wavelengths (respecting phase matching conditions); and
an infrared radiation detector.
Advantageously, the optical means comprise a multi-pass cell in which the light beam passes several times through the gaseous medium to be analyzed, to increase the measurement sensitivity.
The optical means may comprise a filter, allowing only infrared radiation to pass.
According to one embodiment of the invention, the system comprises means of checking the temperature and temperature compensation, in order to keep the laser diodes at a constant temperature.
According to one variant of the invention, the optical means and the storage and processing means are connected by radio or optical fibers.
The electrical power supply means for the device may consist of a battery so that the system is self-sufficient.
According to one particular embodiment of the invention, the double refraction crystal may for example be a gallium selenide crystal.
The system according to the invention may be associated with one or several other identical systems in a network.